Beamforming is a signal processing technique used in sensor arrays for directional signal transmission or reception. Spatial selectivity is achieved by using adaptive or fixed receive/transmit beam patterns.
One medical application that uses beamforming is ultrasound diagnostics. Ultrasound energy is focused at target tissue by a transmit beamformer, and ultrasound energy modulated and returned by the target tissue is focused by a receive beamformer. The receive beamformer may provide signals for generation of B-mode images, color Doppler or spectral Doppler information representing the target tissue, or combinations thereof. Such beamforming systems can provide real-time, cross-sectional (tomographic) 2D images of human body tissue, or the tissue of another subject.
In traditional ultrasound imaging systems, custom application-specific integrated circuit (ASIC) devices perform the beamformer computations. As part of the software beamforming solution, apodization which involves application of appropriate gains to each data channel (each including a dedicated transducer element) is needed to shape the incoming signals in each data channel. This shaping reduces the grating side lobe effects in the beamformed signal that stem from lateral pressure amplitude variations and transducer element spacing.
Other features are desirable in the signal processing data path including dynamic aperture control and data normalization. Dynamic aperture control functions to maintain a desired aperture over the entire ultrasound scanline by dynamically adjusting the number of active transducer elements. Regarding normalization, since a different number of transducer elements may be used at each sample number n (each sample number n corresponding to a different time instant), normalization can account for this scaling effect to maintain a constant signal level for the beamformed signal.
In most ASIC beamformer system designs, the aperture control is handled via analog switches that enable selection of specific combinations of channels of data as a function of a desired aperture. FIG. 1 schematically depicts a simplified block diagram depiction of a conventional ultrasound beamformer system 100 for imaging target tissue that includes switch-based time delay steering and focusing of received echo signals 14 from echoes returning from targets. System 100 comprises a receive array 10 comprising a plurality of transducer elements 11, shown configured as a linear array.
Dynamic aperture control is shown schematically in FIG. 1 as a plurality of pairs of receiver channel switches 16a-16e which are closed in sequence during the echo reception period by an aperture control circuit 17. Aperture control circuit 17 dynamically selects which of the transducer elements 11 are active transducer elements at any given sample time (instant). For each active transducer element 11 while in the receive mode a data channel is established to move target data from the transducer element 11 to signal processing elements in the beamformer. Receiving channel switches 16a-16e are analog electronic switches. Data channels are associated with each of the switches 16a, 16b, 16c, 16d and 16e. 
The data from the respective active transducer elements 11 in each data channel is delayed by respective processing channel time delays comprising the steering delays and focusing delays shown in FIG. 1 during the echo reception period, which acts to dynamically focus the received signal echoes. Following application of the channel time delays, although not shown in FIG. 1, the respective delayed channel signals may be apodized so that each received channel signal is scaled by a desired value using an apodization factor to reduce the grating side lobe effects in the later formed beamformed signal as described above. The apodized signals are then summed by a summing amplifier 15 to produce a beamformed signal shown as a “focused echo signal” in FIG. 1. A plurality of focused echo signals can be used to form a scanline and a plurality of scanlines can be combined to form an image of the target tissue on a suitable display device.
Although not shown, the focused echo signal output by summing amplifier 15 can then optionally be normalized at each sample time n. Normalization can be used to keep the average signal level substantially the same (constant) over the entire scanline over the sample instants n.
The type of dynamic aperture control implemented by conventional ultrasound beamformer system 100 can be difficult to synchronize in terms of the number of active channels and sample time, especially if the implementation is analog. As a result of this synchronization difficulty, determining the data normalization needed to keep the average signal level the same (constant) along the entire scanline as a function of sample time generally becomes even more problematic.